Introduction
In order to grow, multiply, and transmit, pathogens obtain resources from their host, and theoretically the resulting within-host infection load is expected to be proportional to the gained resources. However, too high rates of within-host infection load may come at the cost of increased virulence, thereby incurring a cost for the pathogen as decreased transmission (Blanquart et al., 2016). Thus, evolutionary theory and experimental studies (de Roode et al., 2008) have established that selection should favour intermediate levels of within-host infection load. This trade-off between within-host infection load and transmission has been proposed to maintain polymorphism in pathogen populations, and to prevent the rise of highly virulent pathogens (Frank, 1992). Trade-offs have been sought as an evolutionary solution to limit disease epidemics and the emergence of pathogen strains with extremely high within-host infection load (Zhan et al., 2015). However, insight on how pathogen within-host infection load links to transmission during epidemics where pathogens may encounter variation in both biotic and abiotic conditions (Susi & Laine, 2013, Blanquart et al., 2016, Dutta et al., 2021) has remained limited (Acevedo et al., 2019). In the wild, the limited evidence for trade-offs may be explained by spatial (Osnas et al., 2015) and host mediated processes (Kubinak et al., 2012). The trade-offs restraining within-host infection load may also occur between other traits i.e. adaptation to abiotic conditions (Mboup et al., 2012) or be context-dependent and become evident in stressful environments (Susi & Laine, 2013).
The drivers of disease evolution and epidemics are rarely limited to the interplay of one host and one pathogen, as in the wild most infections occur as coinfections whereby multiple pathogens are simultaneously infecting the same host (Tollenaere et al., 2016, Telfer et al., 2010). Coinfection may fundamentally change pathogen host exploitation strategy in order to outcompete other pathogens sharing the same limited resource (de Roode et al., 2005, Alizon et al., 2009, Alizon & van Baalen, 2008). Thus, it has been suggested that coinfection is an important driver of disease evolution (Alizon & van Baalen, 2008, Alizon et al., 2009). Experimental approaches have measured increased within-host infection load(Bell et al., 2006) and transmission(Susi et al., 2015b, Susi et al., 2015a) under coinfection but there are also exceptions to this trend (Orton & Brown, 2016). Overall, it is well established that the pathogen within-host infection load may change under coinfection, but studies explicitly testing trade-offs between within-host infection load and transmission under coinfection are rare, and evidence remains mixed (Suffert et al., 2016, Sacristan & Garcia-Arenal, 2008). Furthermore, experiments have often been conducted using strains of the same pathogen species, although interspecific interactions among pathogen species are likely to play an important role, as individual hosts often support diverse pathogen assemblages(Susi et al., 2019, Dallas et al., 2019, Telfer et al., 2010).
While intraspecific coinfection is a pre-requisite of outcrossing for many pathogens (Suffert et al., 2016), theory predicts the intensity of competition to increase as relatedness decreases (Alizon et al., 2013). Across plants (Tollenaere et al., 2016, Tollenaere et al., 2017), animals (Telfer et al., 2010) and humans (Lawn et al., 2006, Chen et al., 2020) inter-kingdom coinfections are common, and they are often suggested to have serious consequences in disease epidemics and disease severity. In particular, it is becoming increasingly clear that viruses are ubiquitous in nature (Munson-McGee et al., 2018, Bernardo et al., 2017), although their true diversity and prevalence in natural populations has been under-estimated for a long time (Wren et al., 2006, Roossinck et al., 2015). The ecological roles of viruses are still poorly understood (Roossinck, 2010, Alexander et al., 2014), but they have the potential to interact with other pathogen species via competition for shared host resources, and via shared effects on host immunity (Huang et al., 2019, Uehling et al., 2017). Thus, it is vital to test how coinfection with pathogens from distant taxa may influence within-host infection load and transmission and their potential trade-offs.
Phomopsis subordinaria is a castrating pathogen that infects its hosts through seed stalks. Here, we investigate trade-offs between within-host infection load and transmission by surveying 260 host plant (Plantago lanceolata ) populations in the Åland Islands, south-west Finland, as well as in laboratory trials. In the laboratory, we challenged P. subordinaria strains with Plantago lanceolata latent virus (PlLV) in order to understand how cross-kingdom interactions affect within-host infection load and transmission potential, as well as their potential trade-off. Specifically, we ask 1) How common is P. subordinaria in the Åland Islands, and is there natural variation in within-host infection load in natural P. lanceolata populations, and 2) Is there a trade-off between within-host infection load and population size (potential proxy for among-host natural transmission) in P. subordinaria ? We hypothesize that high within-host infection load comes with a cost of lower transmission, measured as pathogen population size. In a laboratory experiment, we tested: 3) Is there a trade-off between laboratory measured within-host infection load and transmission potential of P. subordinaria ? We hypothesize that high within-host infection load is costly with respect to transmission potential. 4) Does coinfection with PlLV alter P. subordinaria within-host infection load and transmission potential? We hypothesize that coinfection increases both within-host infection load and transmission potential.